The calendering process in paper making involves passing a paper web through a nip or nips formed between one or more pairs of rolls. The paper is thereby made denser or flattened to form a smoother surface while the thickness of the paper is reduced. The apparent density of the resulting web is calculated with Equation 1:Apparent Density=Basis Weight÷Caliper.   (1)
Basis weight here is given in pounds per ream at standard TAPPI conditions (50% RH, 72° F.), where a ream equals 3300 ft2 (500, 25″×38″ sheets) and caliper is the paper thickness measured in thousandths of an inch (or caliper points). The term, bulk, is occasionally referred to, which is defined as the inverse of density. Paper making technology is constantly improving the ability to achieve uniform density.
The present invention addresses an undesirable coated paper surface property related to non-uniform density known in the art as gloss mottle. The term “gloss mottle” refers to variations in specular reflectance from the surface of the sheet. See, e.g., Gloss Sensor Complements Printing Probe, Pap. Film Foil Converter, Vol. 65, No. 3, March 1991, p. 36; P. Mehts, K. Johnson, and D. Wolin, A New Method of Measuring Gloss Mottle and Micro-Gloss, IS&T's NIP-17, International Conference on Digital Printing Technologies, Fort Lauderdale, Fla., October 2001; p. 714-717. The contents of these documents are hereby incorporated by reference. The measurement technique used here, described in a later section, is the Tobias Gloss Mottle test. (Tobias Mottle Tester, Tobias Gloss Mottle Index, Tobias Associates, Inc., Ivyland, Pa.) A critical factor related to the cause of gloss mottle is non-uniform fiber mass distribution in the sheet. Uniform fiber distribution becomes particularly difficult to achieve at higher basis weights. This is due to the natural tendency of pulp fibers to form fiber bundles or “flocs” which are harder to disperse as the basis weight increases. Generally, individual flocs are several millimeters across in the plane of the sheet and have small regions between them that contain somewhat less fiber. Some degree of “flocciness” is present in all papers and results in thickness variations on a relatively small scale. The paper maker must decide how best to expend resources to overcome this effect to improve quality.
High quality glossy coated papers are typically calendered both prior to and after the coating process. The first calendering step usually involves the use of one steel nip, but multiple steel nips or alternate steel and soft nips may also be used. The nip pressure typically ranges from about 7,000 psi to 16,000 psi. This step produces an apparent density for the base stock that ranges between about 11-15 lbs/ream per caliper point. Despite pre-calendering, the base sheet is still characterized by lower hills formed by flocs and valleys positioned between the flocs. In other words, the floc-scale roughness of the base sheet is improved only somewhat by crushing fiber bundles or flocs.
The subsequent coating process further improves the sheet surface. A particularly useful method for coating the base sheet is blade coating, which uses a blade to meter off excess applied coating while the coating is still in the wet state. The blade coating approach levels the surface by filling in valleys or low spots between flocs, but leaves a thinner amount of coating on top of the flocs or high spots. However, water absorption from the wet coating first expands the flocs. Although the blade coating processes fills in the surface roughness, the coating also typically shrinks upon drying such that the original contours of the sheet are still present to some extent. See, e.g., P. Lepoutre, W. Bichard, J. Skowronski, TAPPI J., December 1986, p. 66-70; G. Engstrom, TAPPI J., August 1992, p. 117-122; R. Urscheler, P. Salminen, Practical Study of Free-Jet Application in Paper Coating, 1998 TAPPI Coating/Papermakers Conf. Proc., May 4, 1998, p. 63-72; P. Salminen, D. Eklund, Wochenbl. Papierfabr., Vol. 120, No. 14, Jul. 31, 1992, p. 572-574; M. Leino, M. Veikkola, A New Board Coating Method, 1998 Coating/Papermakers Conf. Proc., May 4-6, 1998, p. 791-806; and U.S. Pat. No. 6,306,461 to Leino et al. The relevant disclosures of the foregoing materials are hereby incorporated by reference. Smoothness and gloss mottle improvements can also be facilitated by multiple coating passes, since subsequent blade coating steps level the low spots further, but still not completely. Moreover, the improvements in gloss mottle and smoothness associated with multiple coatings require the use of additional coaters and drying systems, which increase the overall material, energy, capital, and operating costs of the process. Increased material waste associated with multiple coaters stations can frequently result in a less efficient, overall process.
Contour coating application methods such as air-knife, metered size press, spray, and curtain coaters may also be used. Contour coating typically involves the application of a relatively uniform coating layer thickness that follows the original contour of the base sheet, but typically does not provide the smoothness that can be obtained with a blade coater. Therefore, although the roughness of the sheet is improved relative to the base stock, there is still room for improvement to both gloss mottle and smoothness.
In accordance with the typical finishing operation, the coated base sheet is passed through a supercalender or hot roll calender to impart the final product gloss and smoothness. Both calender types consist of nips formed between a steel roll and a soft roll. Hot roll calenders typically have one or two nips per sheet side. Supercalenders usually have several nips. The finishing step is typically performed at nip pressures from about 2500-8000 psi and the soft rolls are relatively hard (>86 Shore D). The conventional finishing processes result in some degree of paper and print gloss mottle since the influence of flocs is still present. The dense spots formed by flocs will receive more local pressure than the low spots between flocs. This non-uniformity thereby produces the gloss mottle as the product is densifed. The typical ratio between the base stock and finishing calender nip pressures is about 10 or less.
A relatively new finish calendering method involves the use of a shoe calender. A smooth, soft synthetic belt passes with the paper web between a hard, stationary element (shoe) and a heated steel roll. This arrangement provides for a much longer dwell time in the nip at low pressure to develop gloss and smoothness while preserving bulk. The process is gaining popularity in the manufacture of uncoated and coated one-side paperboard grades. The shoe calender yields very good results for gloss mottle, but surface smoothness on a fine scale customary of printing papers coated on both sides are not typically achieved without further refinements to the base paper.
Papers produced in accordance with conventional methods have a Tobias mottle index of about 550 or greater on a 60-1700 scale, where more visually perceived gloss mottle is associated with higher Tobias mottle values. The paper smoothness can be characterized with the Parker Print Surf (PPS-S10=soft backing, 10 kg) method. ISO 8791-4:1992, Paper and Board—Determination of roughness/smoothness (air leak methods), Part 4: Print-surf. The contents of which are hereby incorporated by reference. Tobias gloss mottle indices and PPS-S10 values for several commercial papers at basis weights above about 120 lbs/ream (3300 ft2 basis) are shown in Table 1.
TABLE 1Tobias Mottle Indices and PPS-S10 for Commercial Coated Papers.Basis Weight,Density, lbs/reamTobias MottlePPS-S10,Samplelbs/3300 ft2per caliper pointIndexmicronsA12416.78791.8B12417.211122.1C13017.28231.7D13617.37701.3E14621.35920.76F14621.47220.79G15316.07522.2H15416.39961.9I18419.76970.85It is apparent from the foregoing that there exists a need in the art for a method of producing a coated paper having reduced gloss mottle. In accordance with certain aspects, the method may provide Tobias gloss mottle values measured on coated fine paper that are below about 600 and PPS-S10 values that are below about 1.2. Furthermore, a need exists for a method of manufacturing paper products that exhibit reduced gloss mottle and excellent smoothness with the potential to reduce coating material and energy costs.